In response to increased demand for higher magnetic storage capacity, areal bit densities approaching or greater than 1 Tb/in2 are being contemplated. The bit size of sub 50 nm required to fulfill this goal is within a range where superparamagnetic instabilities affect the lifetime of stored data. Superparamagnetic instabilities become an issue as the grain volume of the recording media is reduced in order to increase the areal density of recorded information. The superparamagnetic effect is most evident when the grain volume V is sufficiently small such that the inequality KuV/kBT>70 can no longer be maintained, where Ku is the magnetocrystalline anisotropy energy density of the material, kB is Boltzmann's constant, and T is absolute temperature. When this inequality is not satisfied, thermal energy can demagnetize the stored bits. As the grain size is decreased in order to increase the areal density, a threshold is reached for a given Ku and temperature T such that stable data storage is no longer feasible.
The thermal stability can be improved by employing a recording media formed of a material with a very high Ku. However, with available materials, recording heads are not able to provide a sufficient or high enough magnetic writing field to write on such a media. Accordingly, it has been proposed to overcome the recording head field limitations by employing thermal energy to heat a local area on the recording media before or at about the time of applying the magnetic field to write to the media in order to assist in the recording process.
Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording media to reduce the coercivity. This allows the applied magnetic writing fields to more easily direct the magnetization during the temporary magnetic softening caused by the heat source. HAMR allows for the use of small grain media, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability, which is desirable for recording at increased areal densities. HAMR can be applied to any type of magnetic storage media including tilted media, longitudinal media, perpendicular media, and patterned media. By heating the media, the Ku or coercivity is reduced such that the magnetic write field is sufficient to write to the media. Once the media cools to ambient temperature, the coercivity has a sufficiently high value to assure thermal stability of the recorded information.
For heat assisted magnetic recording, an electromagnetic wave of, for example, visible, infrared, or ultraviolet light can be directed onto a surface of a data storage media to raise the temperature of a localized area to facilitate switching. Well known optical waveguides such as solid immersion lenses (SILs), solid immersion mirrors (SIMs), and mode index lenses have been proposed for use in reducing the size of a spot on the media that is subjected to the electromagnetic radiation. Due to diffraction limited optical effects, SILs, SIMs, and mode index lenses alone are not sufficient to achieve focal spot sizes necessary for high areal density recording. Metal pins and other near field transducer (NFT) designs are positioned at the focal point of the waveguide and used to further concentrate the energy and direct it to a small spot on the surface of the recording media.
One of the problems in achieving high areal density with HAMR is magnetic transition curvature. A magnetic transition is formed on the magnetic media between two adjacent portions of the magnetic media that have different polarities. Transition curvature is the variation in the location of the magnetic transition as a function of the cross-track position. In HAMR, especially when media coercivity is high, the interaction between the magnetic field profile of the write pole and the thermal profile of the NFT results in a forward-curved transition. The curvature of the magnetic transition or the transition curvature decreases the areal density of the recording media. Further, the reader, which reads the information written, and its shields are flat. Reading a curved transition with a flat reader produces extra noise and reduces the signal-to-noise ratio (SNR) and the bit error rate (BER).